The invention relates to copolymers of tetrafluoro-ethylene and perfluorobutylethylene produced by aqueous dispersion polymerization.
Many prior patents disclose techniques for the dispersion polymerization of tetrafluoroethylene, and variations thereof. The dispersion polymerization of tetrafluoroethylene produces what has come to be known as xe2x80x9cfine powderxe2x80x9d resins. In such a process, sufficient dispersing agent is introduced into a water carrier such that, upon addition of tetrafluoroethylene in the presence of a suitable polymerization initiator and upon agitation, and under autogenous tetrafluoroethylene pressure of 10 to 40 kg./cm2, the polymerization proceeds until the level of colloidally dispersed polymer particles is reached and the reaction is then stopped. See, e.g., U.S. Pat. No. 4,016,345 (Holmes, 1977).
Tetrafluoroethylene powders have also been produced by a process of suspension polymerization, wherein tetra-fluoroethylene monomers are polymerized in a highly agitated aqueous suspension in which little or no dispersing agent may be employed. The type of powder produced in suspension polymerization is termed xe2x80x9cgranularxe2x80x9d resin, or xe2x80x9cgranular powderxe2x80x9d. See, e.g., U.S. Pat. No. 3,655,611 (Mueller, 1972).
For both types of polymerization processes, copolymerization of tetrafluoroethylene with various fluorinated alkyl ethylene comonomers has been described. See, for example, U.S. Pat. No. 4,792,594 (Gangal, et al., 1988). The present invention relates to the aqueous dispersion polymerization technique wherein the product of the polymerization reaction is the copolymer of the invention dispersed within an aqueous colloidal dispersion. This process, generally, is one in which tetrafluoroethylene monomer is pressured into an autoclave containing water and certain polymerization initiators along with paraffin wax to suppress coagulum formation and an emulsifying agent. The reaction mixture is agitated and the polymerization is carried out at suitable temperatures and pressures. Polymerization results in the formation of an aqueous dispersion of polymer. The dispersed polymer particles may be coagulated by techniques known in the art to obtain fine powder polymer. When fluorinated alkyl ethylene comonomers are introduced into the polymerization, it is known that the comonomer reacts much faster than tetrafluoroethylene monomer, and comonomer addition rate is important to the distribution of comonomer achieved in the copolymer. When this comonomer is added as a single precharge, the comonomer is found in polymerized form mostly in the core or interior of the polymer particles. The comonomer may also be injected through some or all of the polymerization process and the injection sequence determines the structure of the shell, i.e., if comonomer is added throughout, it will reside throughout the outer shell of each copolymer particle.
Various prior patents have disclosed variations on techniques for the homopolymerization of tetrafluoro-ethylene and for the copolymerization of other monomers with tetrafluoroethylene. Among those are included U.S. Pat. No. 4,576,869 (Malhotra, 1986) and U.S. Pat. No. 6,177,533B1 (Jones, 2001). Within those references are contained certain procedures which have become, more or less, accepted procedures for determining certain defining and delineating properties associated with tetra-fluoroethylene homopolymers and copolymers. Among those properties are:
(a) the Standard Specific Gravity (SSG), measured by water displacement of a standard molded test specimen, in accord with ASTM D-1457-90;
(b) Raw Dispersion Particle Size (RDPS), determined by spectrophotometry or other suitable technique. See, e.g., U.S. Pat. Nos. 4,016,345 and 4,363,900. The measurements herein were obtained by laser light scattering using a Brookhaven 90 plus instrument.
In the cited prior patents, and almost universally, the SSG of a homopolymer specimen has come to define its molecular weight, with the relationship being inverse, that is, a high molecular weight (MW) corresponds to a low SSG and, generally, the lower the SSG, the higher is the molecular weight. Addition of comonomer into the polymerization process may also reduce SSG and, for resins modified with comonomer, SSG may be used to infer variations in molecular weight at a given constant comonomer level.
For fluoroethylene fine powder polymers, generally, their RDPS range from about 0.175 microns and below to about 0.325 microns. These fine powder resins are known to be useful in paste extrusion processes and in stretching (expansion) processes in which the paste-extruded extrudate, after removal of extrusion aid lubricant, is stretched rapidly to produce porous, strong products of various cross-sectional shapes such as rods, filaments, sheets, tubes, etc. Such a stretching process is disclosed in U.S. Pat. No. 3,953,566 (Gore, 1976), assigned commonly with the instant invention.
Heretofore it has generally been accepted that, for tetrafluoroethylene homopolymers and copolymers of the dispersion type, it is difficult to achieve a resin which combines both desirable properties of small particle size (RDPS) coupled with a high molecular weight (MW). Expressing the same conclusion in a different, equivalent way, it is generally accepted that a dispersion resin possessing a small raw dispersion particle size (RDPS) and a low standard specific gravity (SSG) has been difficult or impossible to achieve.
This invention provides a dispersion type copolymer of tetrafluoroethylene and perfluorobutylethylene comonomer which possesses a heretofore unachieved combination of small fundamental resin particle size (RDPS) coupled with a low SSG (high MW).
A polymerization process for producing a tetra-fluoroethylene copolymer, and the copolymer produced thereby, are provided. The copolymer is of the dispersion/fine powder type and contains polymerized tetrafluoroethylene monomer units and co-polymerized perfluorobutylethylene comonomer units in which the primary particles are believed to have a core and shell structure and the polymerized comonomer units are present in an amount from 0.02% by weight to 0.6% by weight, based upon total copolymer weight. The copolymer has a raw dispersion primary particle size (RDPS) in the range between 0.175 microns to and including 0.203 microns coupled with a standard specific gravity (SSG) of less than 2.143. Preferably the copolymer has comonomer units present in an amount from 0.05% by weight to 0.5% by weight and the RDPS is within the range between 0.178 microns and 0.200 microns coupled with a SSG of less than 2.140.
The copolymer may be dispersed within an aqueous dispersion and/or may be in the form of fine powder.
The process of the invention is characterized in that the copolymerization reaction is catalyzed by potassium permanganate initiator and the entire reaction is carried out in the absence of any multivalent ionic strength enhancer, such as zinc chloride. The addition of initiator is stopped well before completion of the reaction, preferably at or before the mid-point of the complete reaction. Also, and preferably, the comonomer is added as a precharge into the copolymerization reactor, although it may be added incrementally and intermittently through a portion of the polymerization reaction process.
A copolymer of tetrafluoroethylene and perfluoro-butylethylene, prepared by an aqueous dispersion polymerization technique, is provided. This copolymer contains a relatively small amount , between about 0.02 weight percent and about 0.6 weight percent, of fluorinated comonomer polymerization units. The copolymer is believed to be comprised of a core-shell structure wherein the polymerized comonomer units reside primarily within the core. The primary particle size of the copolymer ranges from 0.175 microns to and including 0.203 microns and the standard specific gravity is less than 2.143. This resin is uniquely suited for use in an expansion (stretching) process, to produce high strength, highly porous tetrafluoroethylene polymeric articles.
The polymers of this invention provide the heretofore unachieved combination of properties wherein the fundamental particle size is very small and this is coupled with high molecular weight. These polymers are produced by a dispersion polymerization process, which is described in detail below and in the examples which follow. It can be seen from those examples, and drawing upon basic principles of dispersion poly-merization of tetrafluoroethylene monomers, in particular, certain processing steps disclosed herein are critical.
More Specifically:
Initiation of Polymerization
The copolymer of this invention is produced by a polymerization process wherein the copolymerization reaction is catalyzed by a permanganate initiator, preferably potassium permanganate (KMnO4), in the absence of any multivalent ionic strength enhancer, and the initiator addition is stopped completely, allowing the reaction to slow down and proceed to completion, at a point between 30% and 80% of the progression of the reaction toward completion. Preferably the initiator addition is stopped at about the mid-point of the reaction, i.e., at 40-65% to completion, and most preferably at about 44% of the completion of the reaction.
The perfluorobutylethylene comonomer is preferably added as a precharge in the reaction or, alternatively, it can be added incrementally only through a portion of the reaction.
Dispersing Agents
Substantially non-telogenic dispersing agents are used. Ammonium perfluoro octanoic acid (APFO or xe2x80x9cC-8xe2x80x9d) is an acceptable dispersing agent. Programmed addition (precharge and pumping) is known and is preferred. Decreasing the precharge can lead to increased primary particle size.
Polymerization Control
It is known that ionic strength affects primary particle size control and dispersion stability. Care must be taken to have a sufficiently stable dispersion to enable completion of the polymerization without coagulating the dispersion and to have a sufficiently stable dispersion to survive transportation from the polymerization vessel to the coagulator. Inorganic salts have been precharged into the polymerization reactor with the intended effect of increasing the primary particle size. Multivalent ions, generally, are more effective in increasing ionic strength. Zinc chloride has been employed together with decreased APFO and intended to control (increase) the primary particle size. In the polymerization reaction of the present invention, however, multivalent ionic strength enhancers, such as zinc chloride, are omitted from the reaction.
It is known that particular attention must be paid to ingredient purity to achieve the desired properties in polymerizations as described herein. Ionic impurities, which can also increase ionic strength, in addition to soluble organic impurities, which can cause chain transfer or termination, must be minimized. It is clearly important to employ ultra pure water in all such polymerization reactions.
Additional Test Procedures
The break strength associated with an extruded and expanded (stretched) beading produced from a particular resin is directly related to that resin""s general suitability for expansion, and various methods have been employed to measure break strength. The following procedure was used to produce and test expanded beading made from the copolymers of this invention:
For a given resin, blend 113.4 g. of fine powder resin together with 32.5 ml. of Isopar(copyright) K. Age the blend for about 2 hours at 22xc2x0 C. in a constant temperature water bath. Make a 1 in. diameter cylindrical preform by applying about 270 psig of preforming pressure for about 20 seconds. Inspect the preform to insure it is crack free. Produce an extruded beading by extruding the preformed lubricated resin through a 0.100 in. diameter die having a 30 degree included inlet angle. The extruder barrel is 1 in. in diameter and the ram rate of movement is 20 in./min. The extruder barrel and die are at room temperature, maintained at 23xc2x0 C., plus or minus 1.5xc2x0 C. Remove the Isopar K from the beading by drying it for about 25 minutes at 230xc2x0 C. Discard approximately the first and last 8 ft. of the extruded beading to eliminate end effects. Expand a 2.0 in. section of the extruded beading by stretching at 290xc2x0 C. to a final length of 50 in. (expansion ratio of 25:1) and at an initial rate of stretch of 100% per second, which is a constant rate of 2 in. per second. Remove about a 1 ft. length from near the center of the expanded beading. Measure the maximum break load of the sample at room temperature (23xc2x0 C. plus or minus 1.5xc2x0 C.) using an Instron(copyright) tensile tester using an initial sample length of 2 in. and a crosshead speed of 12 in./min. Measure in duplicate and report the average value for the two samples. This procedure is similar to that described in U.S. Pat. No. 6,177,533B1. The expansion here is carried out at 290xc2x0 C. instead of 300xc2x0 C.